1. Field of the Invention
The present invention relates to a densitometer using a microwave, and more particularly to a densitometer using a microwave which can measure density of suspension (suspended matter) or dissolving matter with high accuracy.
2. Description of the Related Art
As a densitometer for measuring density of suspension (suspended matter) such as sludge and pulp, a densitometer using an ultrasonic wave has been conventionally known. FIG. 1 shows such a conventional densitometer using an ultrasonic wave.
As shown in FIG. 1, a detection pipe 3 is provided between a pipe 1A and a pipe 1B through sluice valves 2A and 2B. On a pipe wall, an ultrasonic transmitter 4 and an ultrasonic receiver 5 are arranged to be opposed to each other so as to contact a measuring fluid flowing in the detection pipe 3. An oscillator 6 is connected to the ultrasonic transmitter 4. An attenuation factor measurement unit 7 is connected to the ultrasonic receiver 5.
According to the above-mentioned densitometer using the ultrasonic wave, the ultrasonic transmitter 4 is driven by the oscillator 6, and an ultrasonic wave is transmitted from the ultrasonic transmitter 4, propagated in fluid of the detection pipe 3, and received by the ultrasonic receiver 5. At this time, strength of the ultrasonic wave is attenuated in accordance with density of the suspension in liquid. The ultrasonic receiver 5 generates an electrical signal in accordance with the receiving strength of the ultrasonic wave. The receiving signal outputted from the ultrasonic receiver 5 is supplied to the attenuation factor measurement unit 7. In the attenuation factor measurement unit 7, there is, in advance, set a working curve showing a relationship between density of suspension and an attenuation factor of the ultrasonic wave in accordance with the density of the suspension. The attenuation factor measurement unit 7 measures density of the suspension within the fluid in accordance with the working curve based on the attenuation factor shown by the receiving signal outputted from the ultrasonic receiver 5.
However, in the above-mentioned densitometer using the ultrasonic wave, the following problems exist.
(a) Since the ultrasonic transmitter 4 and the ultrasonic receiver 5 contact fluid, suspension is adhered to their contact surfaces, and this becomes a factor causing a measuring error. Due to this, it is necessary that the contact surfaces of the ultrasonic transmitter 4 and the ultrasonic receiver 5 be cleaned. Particularly, suspension such as sludge is easily adhered thereto, such that cleaning must be frequently performed.
(b) The above problem can be solved by providing the ultrasonic transmitter 4 and the ultrasonic receiver 5 on the outside of the detection pipe 3. However, in this case, the thickness of a portion of the detection pipe 3 where ultrasonic transmitter 4 and the ultrasonic receiver 5 are located must be thinned in order to decrease the attenuation of the ultrasonic wave in that wall portion of the detection pipe 3. Due to this, there is a problem in strength and durability. Also, the detection pipe 3 is subject to influence of vibration, and this becomes a factor causing a measuring error.
(c) Moreover, as compared with fluid, the attenuation factors of the ultrasonic wave is extremely large in gas. Therefore, if a gas bubble is mixed in the fluid, the attenuation of the ultrasonic wave in gas becomes larger than the attenuation due to suspension. As a result, the density of the measuring fluid cannot be measured. Or, the measuring result having an apparent high density will be obtained.
To solve the above problem, in this type of the densitometer using the ultrasonic wave, there is a densitometer having a bubble erasing mechanism. In the densitometer having a bubble erasing mechanism, a fluid to be measured is introduced into a pressure bubble erasing chamber at a predetermined sampling period. The inside of the pressure bubble erasing chamber is pressurized, and the bubble is dissolved. Thereafter, the density of the suspension within the fluid to be measured is measured by the ultrasonic wave. However, even in this type of the densitometer, since fluid is sampled at the predetermined sampling time, a continuous measurement cannot be performed. Moreover, it is needed that a mechanical movable mechanism be provided since sampling and pressure are performed. Therefore, reliability of the operation is low.
(d) Moreover, the above-mentioned densitometer using the ultrasonic wave uses the fact that the ultrasonic wave is dispersed by the measuring matter and attenuated. Due to this, this type of the densitometer cannot be easily applied to a case that the matter is completely dissolved in fluid.
An object of the present invention is to provide a densitometer using a microwave which can measure density of suspension with high accuracy without causing the disadvantages of a densitometer using an ultrasonic wave.